Fix memory configuration

- vm_definiton.rb
 * always use :memory attribute as in bytes
 * fixed behavior ignoring config value stored in :memory attribute
   but @memory is used to create domain
   that is set by box's definition.
 * remove @memory
- libvirt_domain.erb
 * introduce memory_size and memory_size embeded variables.

Signed-off-by: Hiroshi Miura <miurahr@linux.com>